A CMP (Chemical Mechanical Polishing) apparatus is a device for chemically mechanically polishing a substrate, such as a wafer. The CMP apparatus is configured to provide sliding contact between a wafer and a polishing pad while supplying a polishing liquid onto the polishing pad to thereby polish a surface of the wafer by a combination of a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of the polishing liquid.
Fine particles, such as polishing debris and the abrasive grains, are attached to the surface of the polished wafer. Therefore, in order to remove such fine particles from the wafer surface, the CMP apparatus typically has a cleaning unit for cleaning the wafer. A typical cleaning unit is configured to clean the surface of the wafer with a jet of pure water. However, it is difficult to remove the fine particles from the surface of the wafer by the jet of pure water.
Thus, there is a method of cleaning a surface of a wafer using a chemical liquid in place of pure water. According to this cleaning method, a zeta potential of the particles is changed by the supply of the chemical liquid onto the wafer, so that an attracting force acting between the wafer surface and the particles is weakened. Thereafter, rinsing water is supplied onto the wafer to wash away the particles from the wafer surface. This cleaning method using the chemical liquid has been often used because of its high removal efficiency of the particles. However, since the chemical liquid contains components which are harmful to human body, use of the chemical liquid entails a heavy burden on its handling and treatment.